In X-ray photography efforts are directed to develop highly sensitive emulsions permitting minimum X-ray exposure for the patient, with a minimum fog level so that diagnosis is not disturbed owing to a lack in image quality. Moreover the properties of the film material should be stable even under severe atmospherical circumstances, e.g. high temperatures and high values of relative humidity, so that consistent results can be obtained from the viewpoint of sensitometry as well as image quality.
As a consequence an important aim in the photographic art in general is to develop emulsion making processes suitable for being used on an industrial scale which yield photographic materials with a maximum speed and a minimum overall fog even after long storage periods of said photographic materials.
On the other hand there is a general trend to enhance the speed of processing, especially in the field of radiography, and therefore interest has been focused on rapid access of radiographs, being vital in diagnosis, with development characteristics as independant as possible from development conditions.
It is generally known for one skilled in the art of photography that the sensitometric values and the image quality of a photographic light-sensitive material do not only depend on the characteristics of the emulsion but are determined also to a considerable extent by the processing conditions, in particular the type of developer selected and the degree of development.
As is generally known, the slope of the characteristic curve of a photographic material increases with increasing degree of development. However once a particular limit has been reached, the slope decreases with higher degrees of development as the fog caused by development rises considerably. The same phenomenon can also occur in short development times with more active developing baths having a minimum concentration of stabilizer.
In both cases fog may be suppressed by adding stabilizers to the emulsion layer. However it is not always evident to use whatever a stabilizer to get the expected effect on sensitometry. In addition, even when a stabilizer seems to deliver specific advantages, its use is not favoured, e.g. for ecological reasons, as it is the case for certain heavy metal salts with toxic properties.
Further it is generally known that the sensitometric properties of a photographic film and their dependence from the processing conditions, are determined to a large extent by the size, the crystal form and the crystal structure of the silver halide crystals, which in their turn are determined by the conditions of precipitation and subsequent ripening of the silver halide crystals.
Regular-shaped silver halide crystals useful in photography e.g. may be prepared by employing a technique known as balanced double jet precipitation wherein separate streams of silver nitrate and alkali metal halide are introduced into a stirred gelatin solution and the process is controlled to regulate the form of the resulting silver halide crystals.
By partially or fully controlling the conditions of temperature, concentrations, sequence of addition, and rates of addition it is possible to grow regular, crystalline and uniform particles with a cubic or octahedral form or any transition form.
Especially silver bromide and silver bromoiodide emulsions with cubic crystal habit as used in radiographic materials are known to have favourable development characteristics with respect to high sensitivity, but with the risk of high fog densities. Fog is intensified as the average diameter of the silver halide crystals increases.
If cubic crystals are showing reduction sensitization to a certain degree, leading to an increased sensitivity, it is generally known that the (100)-crystal faces are particularly sensitive to fog. On decreasing pAg during precipitation for increasing reduction sensitization an increase in fog results. The effects of the widely used 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene (TAI) and analogous nitrogen-containing compounds as well as e.g. 1-phenyl-5-mercaptotetrazole and the like as stabilizing agents are unsatisfactory for reducing this fog. Under the influence of humidity and heat the fog level may increase dramatically as is known from experience by the investigators. Hitherto only mercury salts could be selected to give an approximatively acceptable result in the most extreme storage conditions. However from an ecological point of view it is no longer acceptable to introduce these highly toxic products in photographic materials as care for the environment becomes more and more important.